Co–Ni–Al alloys were fabricated by melt-spinning technique. The martensitic transformation temperature of the alloys was found to increase with increasing valence electron concentration. XRD analysis revealed that the β single-phase of Co39Ni33Al28 partially decomposed at temperatures above 573 K by precipitating an hcp-(Co,Ni) phase. Heating to above 973 K eliminated the hcp phase and resulted in a two-phase (β + γ) structure. The decomposition of the β phase was also detected by thermomagnetic measurement. With continuous heating to 1100 K, the magnetization of the alloy was found to decrease rapidly up to 550 K, which is attributed to the Curie transition of the β phase, then increases within the temperature range of 550–780 K before finally diminish again upon heating to above 1000 K. Corresponding to the phase decomposition, the magnetic characteristic also changed. Annealing between 673 and 873 K, where the hcp phase appeared, enlarged the coercive force from 793 to 2373 Oe. Increasing the heat treatment temperature to above 973 K led to a rapid decrease of the coercive force again.